Ferrous metal chips, such as cast iron or steel machining chips are an abundant and low cost source for foundary production of cast iron. These chips however are not directly amenable to remelting in either conventional cupolas or furnaces because of their low bulk density. This material if charged into the top of a conventional melting cupola would be carried off by the high velocity top gases exiting the cupola before being heated to a temperature sufficient for melting. If charged into an induction type melter the loose chips would float on the bath surface and oxidize before melting. The resultant iron oxide represents both a loss of iron unit yield and an undesirable constituent (wustite) which is detrimental to optimum furnace refractory performance. As a result the chips must be compressed into briquettes prior to use in either the cupola or the induction melter.
A conventional cupola is essentially a cylindrical vertical shaft furnace filled with alternate layers of coke, limestone and scrap iron or steel which are charged through ports proximate the top of the furnace. Preheated air, having a temperature of about 1000.degree. F. is injected through tuyeres at the bottom of the furnace. The hot air or wind burns the charged coke and provides heat required for the melting of the charged metal. A small amount of the carbon contained in the coke dissolves into the molten iron to provide the required carbon content for cast iron, approximately 3 to 3.5%. As the combustion gases move vertically upward through the furnace heat is transferred in a countercurrent manner to the downward descending layers of charged material. Thus it can be seen that if a low density charge material such as machining chips were charged into the top of the cupola they would be caught in the gas stream exiting the top of the furnace.
The primary function of the furnace or cupola is to melt iron or steel scrap. The basic design of a cupola is that of a vertical shaft furnace having an outer steel shell supporting a refractory lining or having a water cooled steel shell. The bottom of the furnace is usually enclosed by two doors which are maintained in the closed position by means of a prop. A sand bed is then layered on top the doors to a depth of about 6 to 10 inches with the upper surface of the sand bed being downwardly sloped toward an iron tap hole. A row of tuyeres for the entry of combustion or blast air is provided about the periphery of the shaft. The tuyeres are connected to a common duct known as a wind box from which they receive the blast air. Charging doors are provided at the upper end of the furnace shaft for the entry of scrap charge and coke.
In operation coke is charged into the furnace to a level of approximately 60 inches above the tuyeres. The shaft is then filled through the charging doors with alternate layers of metallic charge and coke. The coke is ignited and blast air is introduced into the coke bed through the tuyeres thereby developing intense heat. The metal at the upper surface of the coke bed melts and trickles down through the hot coke to collect on the sand bottom. The column of charged material descends to replace the metal melted and a fresh layer of coke replenishes coke which was burned in the bed to melt the charge. This process continues as long as the air supply is continued and coke and metallic charge are added. Molten slag is also formed and floats on the surface of the molten iron collected at the bottom of the furnace. Flux, usually limestone, is added to fluidize the slag which is formed. The hot combustion gases which include carbon monoxide, a reducing gas, move upwardly and countercurrently through the layers of coke and metal scrap and the exit the top of the furnace stack. The passage of these combustion gases preheats and prereduces the materials which have not undergone melting. The gases exiting the top of the furnace are directed into conventional air pollution control devices for cleaning and ultimate release into the atmosphere.
The blast air which is used in the furnace may be either at ambient temperature or at an elevated temperature. This is referred to as cold blast and hot blast, respectively. While the furnace will perform it basic function of melting iron when a cold blast is used, numerous benefits are derived when hot blast is implemented during furnace operation. Hot blast increases the iron temperature, decreases coke comsumption per ton of iron melted, increases the melting rate, and provides secondary benefits in the form of lower melting losses, less sulfur pick up, and increased ability to use low carbon raw materials. Heating of the blast air is accomplished by use of either a separately fired blast air heater or a recuperative blast air heater, the latter combusting the carbon monoxide in the top gases to heat the blast air. The use of hot blast also allows better control over the chemistry of the melted produst. The temperature of the blast air has a direct rapid and quantitative effect the iron tapping temperature and in turn on the carbon content. Thus variation in the temperature of the combustion air will vary the temperature of the melted metal which in turn will vary the percentage of carbon in the melted product.
A more detailed look at the construction and operation of melting furnaces or cupolas may be found in Metals Handbook, 8th Ed., Metals Park, Ohio, American Society for Metals, 1970, Vol. 5, Forging and Casting, pp. 335-348.
Because the scrap metal in the furnace must move countercurrently to the flow of combustion gases it must be of a size sufficient to ensure its downward progress in the furnace. Because of this the scrap metal typically has a weight in the range of 50 to 150 pounds. Thus, where metal chips or fines are used, they are compressed into briquettes so that they have sufficient weight to travel downwardly through the furnace without being entrained in the countercurrent gas flow. In U.S. Pat. No. 4,160,867 issued July 10, 1979 entitled "Method and Apparatus for Melting Machining Chips", U.S. Pat. No. 4,214,736 issued July 29, 1980 entitled "Acr Heater Melting System" and U.S. Pat. No. 4,311,519 issued Jan. 19, 1982 entitled "Melting Furnace for Granulated Metal" examples of furnaces for the melting of metal chip feed are presented. The first two patents utilize electric arc heaters to directly heat the molten metal bath in the furnace and to heat a stock feed gas used for pre-reduction of the metal chips. However, each of these designs suffer from carryover of fine and small metal chips due to the countercurrent gas flows in the furnace; thus requiring briquetting of the feed materials.
Because metal fines and chips are abundant and low cost it would be advantageous to have a melting furnace which could utilize this material without briquetting.